(1) Field of the Invention
The present invention relates to change detection. More particularly, the present invention relates to methods and systems to detect changes in sonar images.
(2) Description of the Prior Art
The placement of sea-floor mines and underwater improvised explosive devices by hostile forces has been recognized as a significant threat to navigation. To successfully and effectively mitigate this threat in harbor waters, ports and waterways, it is required to conduct periodic sonar image surveys to try and detect any newly inserted objects.
Object detection can be accomplished by comparing newly obtained images with historical imaging data using coherent/incoherent sonar image correlation. By correlating co-registered temporally separated images using amplitude and phase information, subtle changes can be identified. Thus, man-made changes in the seafloor that are associated with the placement of mines or other objects implanted on the seafloor can be identified.
Change detection through coherent/incoherent sonar image correlation is widely regarded as a difficult problem due to the tendency of sonar images to change dramatically in response to aspect angle and the difficulty of precisely co-registering the repeat-pass survey images. Thus, a need has been recognized in the state of the art to develop innovative motion estimation techniques in order to improve reliability and fidelity of synthetic aperture sonar (SAS) imagery.
There is a further need to provide a means to increase the operation envelope of SAS systems by relieving environmental limitations. In addition, there is a need to reduce costs by developing motion estimation techniques that can be completed without inertial navigation system (INS) data.